Analog to digital (A/D) converters are used in a wide variety of electronic circuits. An analog to digital converter is an electronic circuit that receives an analog input signal and produces a digital output signal. The digital output signal is produced based on samples of the analog input signal taken over time.
In some circuits, it is desirable to maintain the peak input voltage of the analog signal at or near the maximum value for the range of input signals accepted by the analog to digital converter. Thus, automatic gain control circuits have been used in conjunction with analog to digital converters. Typically, the automatic gain control circuit monitors the input to the analog to digital converter. The automatic gain control circuit further generates a feedback signal based on the monitored input signal. The feedback signal is provided to an amplifier to control the level of the input signal to the analog to digital converter. This feedback signal attempts to keep the peak voltage level of the input signal at or near the full-scale value of the input for the analog to digital converter.
Conventionally, the feedback signal is generated using analog circuitry. For example, such feedback loops typically include one or more of the following analog control blocks: a log amplifier, a summing amplifier, an integrator, and a differentiator. Unfortunately, these analog feedback control loops typically suffer from the so-called "clip" effect. This means that when the input signal exceeds the full-scale input for the analog to digital converter, the control circuitry is unable to quickly reach steady-state operation. Some control loops attempt to use digital circuitry. However, these digital control loops typically suffer from an additional problem relating to the quantization of the control word used to adjust the gain of the input signal for the A/D converter. In this situation, when the control loop reaches a steady-state, it tends to introduce an oscillation effect on the input signal to the A/D converter.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved automatic gain control circuit for an analog to digital converter input.